The goal of this grant application is to generate and test transgenic mouse lines containing ligand regulated Cre recombinase driven by 3.6 and 2.3 kb fragments of the rat Collal promoter. Currently available information T2 indicates that a tamoxifen regulated Cre, Cre-ER , is probably the most appropriate for our studies, Unmodified Cre recombinase constitutively catalyzes deletion of DNA between loxP sites, however Cre-ER T2 is inactive in the absence of the estrogen analogue tamoxifen. Constructs containing the 3.6 and 2.3 kb Col I al promoter driving Cre-ER T22 will be produced and used to generate transgenic mice. Testing of these transgenic mouse lines will be carried out by crossing them with a ROSA 26 Cre reporter mouse strain in which expression of B-galactosidase (beta-gal) or green fluorescent protein (GFP) is dependent on Cre activity. It is expected that reporter gene expression will be specific to the cell types that express the Collal promoters, and dependent on injection of tamoxifen. We will characterize the minimum dosage and time course of tamoxifen treatment needed to optimally induce the reporter gene. We will also characterize the effects of long-term tamoxifen treatment at this dosage on bone metabolism. The purpose for developing these lines is to enable tissue specific and temporally regulated inactivation of genes that are expressed in osteoblasts and are believed to play important roles in regulating osteoblast function, but whose role in adult animals is not understood. By allowing normal expression of a gene during early development and inactivating the gene in adult osteoblasts, we can learn about the specific role of the gene in adult bone metabolism.